The present invention relates to an implantable cardiac stimulation device capable of delivering both high and low voltage therapies for treating bradycardia, tachycardia, and fibrillation. The present invention relates more specifically to an implantable cardiac stimulation device possessing automatic sensitivity control and beat-by-beat automatic capture.
Implantable cardiac stimulating devices include pacemakers and cardioverter defibrillators (ICDs). A primary function of pacemakers is to detect and treat incidents of a slow heart rate, known as bradycardia, or no heart rate, known as asystole. A primary function of ICDs is to detect and treat incidents of an excessively high heart rate, known as tachycardia, or incidents of fibrillation.
Combined pacemaker/cardioverter defibrillators are commercially available for treating both bradycardia, and tachycardia or fibrillation. Such a combined cardiac stimulating device is coupled to the patient""s heart through transvenous leads which are used to sense electrical signals from the heart, and deliver both low voltage and high voltage electrical therapy to the heart.
The pacemaker circuitry generally includes sensing circuitry for sensing cardiac electrical activities in order to detect intrinsic electrical depolarizations of the cardiac tissue that cause contraction of the respective heart chambers. In the atria, detection of a P-wave indicates atrial contraction, and in the ventricles detection of an R-wave, also known as a QRS complex, indicates ventricular contraction.
If detection of an intrinsic P-wave or an R-wave does not occur within a given interval of time, generally referred to as the xe2x80x9cescape interval,xe2x80x9d the heart rate is determined as being too slow. A stimulation pulse is then generated by the pacemaker circuitry and delivered to the appropriate heart chamber at the end of the escape interval in order to stimulate the muscle tissue of the heart to contract, thus maintaining a minimum heart rate. The duration of the escape interval corresponds to some base pacing rate, for example an escape interval of 1,200 msec would maintain a base pacing rate of 50 heart beats per minute.
The electrical depolarization caused by delivery of a pacing pulse is known as an xe2x80x9cevoked response.xe2x80x9d An evoked response occurs when the stimulating pulse is of sufficient energy to cause depolarization of the cardiac tissue, a condition known as xe2x80x9ccapture.xe2x80x9d The minimum stimulating energy required to capture a chamber of the heart is known as xe2x80x9cthreshold.xe2x80x9d
Modern pacemakers often include a feature known as xe2x80x9cautomatic capture.xe2x80x9d When automatic capture is implemented, the pacemaker circuitry detects the evoked response following delivery of a pacing pulse in order to verify that capture has occurred. If no evoked response is detected, the pacing pulse may have been of insufficient energy to capture the heart; therefore, a high-energy back-up pacing pulse is quickly delivered to the heart in order to maintain the desired heart rate. A threshold detection algorithm is next invoked in order to re-determine what minimum energy is required to capture the heart. The pacing pulse energy is then automatically adjusted to this new threshold value plus some safety margin. As long as an evoked response is detected following a pacing pulse, that is as long as capture is verified, pacing will continue at the set rate and pulse energy. Hence automatic capture improves pacemaker performance in at least two ways: 1) it verifies that the stimulation therapy delivered has been effective in pacing the heart chamber, and 2) it improves battery energy longevity by determining the lowest stimulation energy needed to effectively capture the heart.
The cardioverter defibrillator circuitry of an implantable cardiac stimulating device monitors the electrical activity of the heart to detect when the intrinsic heart rate exceeds a defined upper rate limit. In the case of tachycardia, a high energy stimulation pulse is usually delivered in synchrony with the heart""s QRS wave in an attempt to terminate the tachycardia, a treatment known as xe2x80x9ccardioversion.xe2x80x9d Synchronized delivery of the high-energy pulse prevents stimulating the heart during the T-wave portion of the P-QRS-T cardiac cycle. During the T-wave portion of the cardiac cycle, the ventricular tissue is re-polarizing and delivery of any kind of stimulation pulse during this time could accelerate the heart rhythm into a faster tachycardia or even into fibrillation.
A serious form of tachycardia is ventricular fibrillation, which is usually fatal if not treated within a few minutes of occurrence. During fibrillation, disorganized depolarizations occur throughout the heart tissue (myocardium) causing the heart chamber to contract in a chaotic way, i.e., fibrillate, resulting in ineffective ejection of blood from the heart chamber. These disorganized depolarizations, also referred to as fibrillation waves, are typically low amplitude signals that occur at an irregular rate. When the cardioverter-defibrillator circuitry detects fibrillation, a high energy shocking pulse is delivered in an attempt to re-coordinate the depolarization of all (or most of) the individual muscle fibers and thus regain coordinated cardiac contractions.
In order to allow detection of both higher amplitude R-waves and low amplitude fibrillation signals, implantable cardioverter defibrillators commonly include automatic gain control or automatic sensitivity control for detecting both high amplitude R-waves and low-amplitude fibrillation signals. Reference is made to U.S. Pat. No. 5,685,315 to McClure et at. for a more detailed description of the use of automatic sensitivity control in cardiac arrhythmia detection, herein incorporated by reference. Automatic gain or sensitivity control allows straightforward detection of cardiac events based on event amplitude crossing of the sensing threshold. An initially higher sensing threshold is applied starting at the end of a refractory period that follows a detected P-wave or R-wave or a delivered pacing pulse. As the gain or sensitivity decays, detection of low amplitude fibrillation waves is possible. The rate at which the detected events occur allows classification of the detected rhythm into bradycardia, normal sinus, low rate tachycardia, high rate tachycardia, or fibrillation.
However, a problem exists for patients having a combined pacemaker cardioverter defibrillator in that the electrogram signal from the ventricular fibrillation may be so low in amplitude that neither the ICD nor the pacemaker sensing circuits sense anything, thus causing the pacemaker portion of the system to release a stimulation pulse. Upon releasing the stimulus, the automatic sensitivity feature the sensing circuits of the stimulation device, if enabled, incrementally increases its sensitivity to its most sensitive setting, in an attempt to sense an R-wave. If a failure to sense an R-wave persists, the diagnosis is xe2x80x9ctrue asystole,xe2x80x9d and the stimulation device will continue to release stimulation pulses at its programmed base pacing rate. However, if the rhythm is truly ventricular fibrillation with an electromyogram signal that is too low to be sensed by either the stimulation device, the stimulation pulses will not be effective. However, the stimulation device does not recognize the ineffectiveness of the stimulation pulses, and will continue to deliver such ineffective stimuli.
Therefore, what is needed is a combined stimulation device or system wherein a proper response to an alleged asystole can occur, and wherein the device can ascertain whether or not a given stimulation pulse is effective, i.e., whether it xe2x80x9ccapturesxe2x80x9d the heart. The automatic capture feature is therefore also desirable in a combined ICD/pacemaker in order to ensure effective stimulation therapy and to increase device longevity by conserving battery energy. The importance of providing automatic capture is described in U.S. Pat. No. 5,350,401, to Levine, which is incorporated herein by reference.
One problem in determining capture is the phenomenon known as xe2x80x9clead polarization.xe2x80x9d Lead polarization is commonly caused by electrochemical reactions that occur at the electrode-tissue interface following delivery of an electrical stimulation pulse. However, the polarization signal, also referred to as an xe2x80x9cafterpotential,xe2x80x9d can corrupt the evoked response signal that is sensed by the sensing circuits. This undesirable situation occurs because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time.
One way to minimize the effect of lead polarization in pacemakers is to sense through a different combination of electrodes than the electrodes used for delivery of stimulation pulses. For example, it is possible to stimulate in a unipolar configuration, using the tip electrode of a bipolar lead as the cathode and the device housing as the anode, and to sense the heart signals in a bipolar configuration using the ring electrode and tip electrode of the same bipolar lead. By using a different electrode combination for sensing than for pacing, saturation of the sensing amplifier due to lead polarization is avoided. However, in combined ICD/pacemaker systems, electrode configuration switching from unipolar pacing to bipolar sensing, or various other configurations, is generally not available for the reason that unipolar pacing in cardioverter defibrillators is not desirable because it may interfere with arrhythmia detection. Pacing stimulation is generally delivered in a bipolar configuration via a tip electrode and a ring and/or coil electrode, the same electrodes that may be used for sensing.
Other methods of reducing lead polarization effects are known such as emitting a pulse of the opposite polarity (discharge pulse) immediately after the stimulation pulse. However, minimizing lead polarization effects alone, does not completely solve the problem of evoked response detection in cardiac stimulation devices combining pacemaking, cardioversion and defibrillation functions.
A refractory period is typically applied immediately following a pacing pulse or a detected intrinsic depolarization. The refractory period primarily prevents the sensing of T-waves, which follow the QRS complex. Over-sensing of T-waves could cause inappropriate tachycardia detection resulting in unnecessary, potentially harmful, therapy delivery. The refractory period is kept as short as possible in order to maximize the window for sensing high rate rhythms, but must be long enough to prevent over-sensing of T-waves. This refractory period therefore, could prevent detection of the evoked response following a pacing pulse.
One way of discriminating between T-waves and R-waves would be to use digitized EGM signals for comparison to depolarization signal templates. For example, a method for automatic capture in an implantable pulse generator can allow for comparison between an EMG signal and a depolarization template after analog-to-digital conversion of these signals. Reference is made to U.S. Pat. No. 5,350,410 to Kleks et al. However, many implantable defibrillators include digitized electrogram storage so that cardiac electrical activity leading up to an arrhythmic episode may be analyzed. Digitized EGM signals are stored in memory and later recalled in order to provide useful diagnostic information. Hence, the microprocessor of the ICD/pacemaker device may be busy collecting and digitizing EGM signals from sources other than the chamber in which a pacing pulse was delivered, making the EGM analog-to-digital converter unavailable for capture verification. Digitized EGM signal detection dedicated to capture verification on a beat-by-beat basis during pacing would require precious microprocessor time and, without additional circuitry, would likely be provided at the cost of disregarding EGM data collection and storage in a combined ICD/pacemaker.
It would thus be desirable to provide an implantable cardiac stimulating device possessing pacing, cardioversion, and defibrillation functions in which capture verification can be performed. Further, it would be desirable to provide beat-by-beat capture verification during pacing operations without additional hardware or circuitry and without extensive consumption of microprocessor time. It would also be desirable to provide a system capable of automatically searching for the capture threshold whenever capture is lost and appropriately adjusting the pacing energy to the new threshold level plus some safety margin, thus providing the advantages of automatic capture in a combined ICD/pacemaker device without introducing any known disadvantages.
The present invention addresses the foregoing problems by providing an implantable cardiac stimulation device possessing pacing, cardioversion and defibrillation functions and automatic capture capabilities, for automatically verifying capture during stimulation operations and, as necessary, automatically delivering back-up stimulation pulses when capture is lost, and subsequently adjusting the stimulation energy to a level safely above that needed to achieve capture.
One aspect of the present invention is a method for detecting the presence or absence of an evoked response following the delivery of a stimulation pulse. This capture verification method advantageously utilizes the automatic sensitivity control hardware commonly included in implanted cardioverter defibrillator systems to allow a straight-forward threshold detection technique, thus avoiding more intensive signal recognition schemes. In the case of no evoked response detection, the circuitry of the stimulation device is notified of the capture failure and is enabled to provide a back-up stimulation pulse in order to sustain the desired contraction rate.
A further aspect of the present invention includes a method for maintaining capture by using a capture search algorithm. Whenever loss of capture is detected, the minimum stimulation energy required to achieve capture is re-determined by delivering test stimulation pulses at progressively increasing or decreasing energy levels, using the capture verification method of the present invention to detect when capture is achieved or lost. In so doing, battery longevity of the cardiac stimulating device is improved by maintaining the lowest energy needed to deliver effective stimulation therapy.
Another aspect of the present invention includes automatic threshold testing that can be invoked on an event-triggered or periodic basis in order to determine the minimum stimulation pulse energy needed to ensure capture. Such a threshold test is implemented in a cardiac stimulation device possessing a method for verifying capture. Using the capture verification method of the present invention, automatic threshold testing in a combined ICD/pacemaker stimulation device is provided.
One embodiment of the present invention is, therefore, an implantable cardiac stimulation device including a method for sensing cardiac events and delivering both high and low voltage stimulation therapies for appropriately treating bradycardia, tachycardia, or fibrillation. One method of therapy delivery includes: 1) sensing for cardiac activity within a cardiac chamber during a defined escape interval; 2) when intrinsic cardiac activity is not detected within the given escape interval, delivering a stimulation pulse for the purpose of stimulating the cardiac chamber to contract at a desired rate; 3) verifying that the delivered stimulation pulse produced an evoked response by sensing during an alert interval following a short refractory period; 4) if no evoked response is detected during the alert interval, sustaining the desired stimulation rate by delivering a back-up stimulation pulse; 5) whenever a back-up stimulation pulse is required, performing a capture search for determining the minimum pulse energy needed to reliably achieve capture, and 6) adjusting the programmed stimulation energy to a level safely above the newly determined capture energy. When the automatic capture function is enabled, the stimulation device initiates a stimulation refractory period, upon the expiration of which, the stimulation device sets a sensing threshold to an evoked response threshold. During the alert interval, the evoked response threshold could be constant or decaying. Concurrently, the stimulation device verifies capture during the alert interval.
Thus, one feature of the present invention is a method for automatically and reliably ensuring that capture occurs during stimulation operations of an implantable ICD/pacemaker (also referred to herein as stimulation device). Another feature of the present invention is to provide reliable threshold testing in an implantable cardiac stimulation device. By providing automatic capture and automatic threshold testing, the stimulation device performance and battery longevity are improved.